The present invention relates to a scanning type probe microscope utilizing AFM (scanning type inter-atomic force microscope) principle and the like.
In AFM, non-contact AFM and MFM (scanning type magnetic force microscope) which are all scanning type probe microscopes, in order to obtain information of the topography and the like of a sample surface, minute forces acting between a probe and the sample surface are detected. In order to detect such minute forces, a so-called optical lever method is utilized in many cases. In this method, a laser beam is directed at a cantilever, or spring arm, and changes in the path of the laser beam reflected from the cantilever are detected.
An apparatus which utilizes the above-mentioned optical lever method has a mechanism to adjust the direction of the irradiating laser beam or the position of the cantilever in two directions, or dimensions, so as to position a laser spot at a desired position on the upper face of the cantilever.
Using the above-mentioned adjustment mechanism, the laser spot position on the upper face of the cantilever is subjected to positional determination and adjustment by confirming a situation of relative movement of the laser spot on the upper face of the cantilever while artificially observing it with an optical microscope, or displaying an image obtained by a CCD camera attached to the optical microscope on a CRT display.
FIG. 3 is a block diagram of a laser driving unit of an AFM measurement control system in the prior art. For automatic adjustment of light quantity, or intensity, a monitor current IM is provided by a monitor photodiode MD having a linear characteristic with respect to laser power and current IM is subjected to current-voltage conversion by a current-voltage converter 101. The converted voltage is compared by a comparator 102 with a voltage corresponding to a desired laser power set value set by a laser power setting circuit 104, and the voltage difference signal from comparator 102 controls a current driving circuit 103 which produces a laser driving current IL which is a function of the voltage difference. In such a manner, owing to the constitution in which the monitor current is fed back to a semiconductor laser LD, to produce a laser light beam 118, the laser output power is held at the set value independently of changes in temperature and the like.
SW1 is a laser oscillation changeover switch, wherein the laser is in an oscillation state when switch SW1 is closed, or there is a state in which the laser is not emitting at all when the switch is open.
The positional adjustment of the laser spot on the upper face of the cantilever by the above-mentioned semiconductor laser is performed in an ordinary sample surface measurement state, namely in a laser oscillation state. However, in the laser oscillation state, the area of the laser spot is approximately the same size as the area of the cantilever, and it is therefore difficult to position the center of the laser spot at a desired position on the upper face of the cantilever. Further, in the laser oscillation state, the laser spot has a high brightness level and a complex shape, and it also becomes difficult to specify the center of the laser spot.